This invention relates to an outboard motor exhaust system and more particularly to an improved catalytic exhaust system for outboard motors.
Because of the compact nature of outboard motors, the design of many of the components of the outboard motor is made quite difficult. A specific area where the size constraints present problems is in the design of the exhaust system for the outboard motor. In many vehicle applications, the exhaust system which transfers the exhaust gases from the engine to the atmosphere can have its length and shape varied within large parameters due to the nature of the vehicle that is powered. However, with outboard motors the exhaust system must be very compact and hence the silencing efficiency and tuning becomes quite a problem. For this reason, it is the normal practice to discharge the exhaust gases to the atmosphere through an underwater exhaust gas discharge under most running conditions. In this way, the exhaust gases are cooled and silenced by the body of water in which the watercraft is operating.
Also, it has been proposed to introduce the cooling water from the engine cooling jacket into the exhaust system so as to assist in the silencing and cooling of the exhaust gases. Cooling of the exhaust gases is important because this not only aids in the silencing, but it protects surrounding components from the heat of the exhaust. This heat transfer is a particular problem because of the compact nature of the outboard motor.
However, with increasing emphasis on protection of the environment, it is necessary or desirable to employ catalytic treatment systems for the exhaust gases. The catalyst can be utilized to remove harmful constituents from the exhaust gases or to transform them into less harmful products before they are discharged. Since the exhaust gases are also delivered to the atmosphere through the body of water in which the watercraft is operating, the control of water pollution also may dictate the desirability of employing catalysts.
However and is well known, the catalyst normally operates at a relatively high temperature to be effective. Also, the material of the catalyst is such that if it is cooled suddenly, the catalyst bed may fracture. Also, any water which may come in contact with the catalyst can pollute the catalyst by leaving deposits when the water evaporates. These problems are particularly acute in conjunction with operation in marine environments. The salt water can leave salt deposits on the catalyst that can foul it and reduce its capability of performing its intended functions.
In order to protect the catalyst from water damage, particularly when considering the fact that the exhaust gases have a discharge outlet that is below the water level, it is desirable to position the catalyst relatively high in the outboard motor. Such high locations will assist in protecting the catalyst. However, if the catalyst is positioned in close proximity to the power head, then the heat transfer problems become particularly acute. This is particularly true since a portion of the protective cowling of the power head is formed from a non-metallic material, such as a molded fiberglass reinforced resin or the like. These materials are quite susceptible to damage under high temperatures.
In order to obtain optimum performance of the catalyst, it should also be located as close as possible to the engine exhaust ports. This also dictates a high placement for the catalyst.
Bearing in mind these problems, it is, therefore, a principal object of this invention to provide an improved exhaust system for an outboard motor.
It is a further object of this invention to provide an improved catalytic exhaust system for such applications.
It is a still further object of this invention to provide an outboard motor exhaust system that employs a catalyst and wherein the catalyst is permitted to operate at a high temperature, but its temperature is insulated from surrounding components of the outboard motor which could be damaged by the heat.
These results could be achieved by placing the catalyst at the upper end of the drive shaft housing. However because of the special limitations already noted, this presents difficulties. Also the catalyst is a component which may require servicing at regular intervals. This is rendered more difficult if it is placed in the drive shaft housing.
It is therefore a further object of this invention to provide an improved arrangement for mounting and servicing a catalyst in an outboard motor exhaust system.